The duplexer as it is known today gained significance around 1988. Before then, it was used for radars in the 50s, 60s and 70s. Some even contained gases. Today, the majority of the duplexers use dielectric materials having a quality factor, which results in a low insertion loss. They also use ceramic materials with a high permittivity in order to miniaturize the passive circuits.
Over the years, the duplexer has evolved as follows:
1953—Use of duplexers with cavities and in the configuration shown in FIG. 1. The cavities were in fact band-pass filters, which could be adjusted mechanically.
1956—The duplexers were made from two hybrids and two tubes. The two tubes are filled with gas and play the role of a switch. These duplexers could not send signals and receive at the same time; they were therefore “half-duplex” applications use mostly in military radars. Twenty years later, no major change had been brought to the duplexers until the appearance of large cellular systems, which required companies to research ways of miniaturizing and optimizing duplexer performance.
1988—Cellular technology requirements increasingly called for more powerful and smaller components. SAW filters (Surface Acoustic Wave) revolutionized this need. These filters convert electromagnetic energy into SAW energy. SAW filters promised to considerably reduce the volume of the duplexers given that they are very small and moreover, they can support higher powers (2 Watts).
1995—Use of SAW filters in GSM cellular band handsets, with the advantage of a low insertion loss and a small volume that could be produced in large volumes. They also made it possible to avoid the use of traditional components: coils, capacitors, etc.
1999—Use of LTCC (Low-Temperature Co-Fired Multilayer Ceramic) technology. This technology made it possible to obtain a smaller duplexer. This duplexer used planar filters in “stripline” with a high permitivity and a volume of 30% compared to the cast solid duplexer, while having the same performance.
2000—Use of two hybrids and two band-stop filters in a chamber filled with liquid nitrogen at a temperature of 80 degrees Kelvin thus offering a low insertion loss by superconductivity. The two band-stop filters were adjusted to reject the receiver frequencies. By using a substrate of LAO (LaAlO3), this duplexer gives a loss by insertion of less than 1.15 dB and all insulation higher than 35 dB.
  Isolation  =            S              Tx        -        Rx              =                            b                      2            ⁢            B                                    a                      1            ⁢            A                              =                        (                                                    S                21                            ·                              S                21                                      +                                          S                24                            ·                              S                31                                              )                ·                  ⅇ                                    -              j                        ⁢                                                  ⁢            ϕ                              
2001—The latest technology of duplexers uses FBAR (Miniature Bulk Acoustic Resonator) technology by Agilent Technologies. This technology makes it possible for the duplexer to occupy less than 20% of the volume of conventional ceramic duplexers.
With the increase in the number of cellular users and the great diversity of their needs, mobile communication has never progressed so quickly. Thousands of dollars are invested in research each year to optimize, miniaturize and find new lower cost solutions in order to fill the needs of an extremely profitable market. Most of the research is therefore centered on increasingly powerful, small an inexpensive electronic parts.
Considering the need to miniaturize the handset components, it is normal to focus on that which takes the largest volume: the duplexer. The duplexer is fundamentally a passive component installed in most full-duplex cellular system. It plays the role of interface between the transmitter, the receiver and the antenna. The ultimate objective of the duplexer is to convey the power coming from the transmitter towards the antenna and to convey the power coming from the antenna towards the receiver. Ideally, no power must pass between the transmitter and the receiver and vice versa. To meet this requirement, the duplexers use analog filters. Thus, in the last twenty years researchers have especially concentrated their effort on the discovery of new filter technologies and not in the development of various topologies of duplexers.
Thus, there is a need in the industry for better performing and cheaper duplexers.